Scanning electrochemical microscopy (SECM) and atomic force microscopy (AFM) are imaging techniques, which can allow the structure or reactivity of a sample surface to be imaged with precision below the micrometer scale. SECM is a type of microscopy that images surfaces using a physical probe. By mechanically moving the probe across a surface, an accurate image of the surface can be mapped. Diameters of existing SECM electrodes are larger than one micrometer and exhibit an electric field that is generally non-localized along the electrode surface. The AFM cantilever tips holding carbon nanotubes (CNTs) are known and implemented in industry and research. Cantilevers of the AFM with the CNTs improve the sharpness in the topographic measurement, but the conductivity of the CNTs is quite uncertain.
Techniques such as utilizing a sharp corner of the cantilevers for the probing tip, using a silicon nitride pyramid tip, and using a sharp single-crystal silicon tip have been able to achieve good results in resolving relatively flat samples with high resolution. Other fabrication techniques such as etching, deposition, or attachment have been used to microfabricate sharp tips.
Anisotropic etching, isotropic etching, dry etching, oxidation sharpening and focused ion beam (FIB) etching have all been used, and deposition methods include techniques using electron beam induced deposition and diamond-like film deposition. Alternatively, attaching a thin probe to an otherwise conventional tip has been implemented by attaching a zinc oxide whisker to the cantilever end, attaching a carbon nanotube (CNT) or bundles of CNTs to an existing silicon etched tip, and attaching a single nanowire to an existing silicon etched tip.
Nanostructures, and in particular, nanowires are an important aspect of facilitating emerging technologies. A major impediment to the emergence of this new generation of electronic devices based on nanostructures is the ability to effectively grow and harvest nanowires and other nanostructures that have consistent characteristics. Current approaches to grow and harvest nanowires do not facilitate mass production and do not yield consistent nanowire performance characteristics.
Metallic nanowire probes can be used for SECM applications because of their excellent conductivity. As integrated circuit devices continue to become smaller and with the rise in importance of nanotechnology, higher resolution surface studies at the atomic level are becoming more necessary. Higher resolution probes require sharper probe tips to minimize the distortion of images.
While CNT probes have so far shown great potential due to their high aspect ratio and small radius of curvature, as well as being chemically stable and mechanically robust, the difficult and low yield process of mounting an individual single-walled nanotube (SWNT) on an existing probe remains a stumbling block. Methods that directly grow a single CNT by surface growth chemical vapor deposition CVD processes have low yields that can be as low as around 10% for individual CVD SWNT tips. In addition, CNT tips may also give rise to artifacts introduced by the probe tip structure.
What is needed is a method to improve the space resolution of SECM, overcome the uncertainty of the electronic conductivity of AFM-CNT tips and enable control of the longitudinal composition distribution of the tips.